Here's data for regions up to Sinnoh. In case of a route that appears in multiple games, the earliest version in which that route appears is taken to be authoritative for the grass-patch count.
Kanto
Route 1: 104
Route 22: 40
Route 2: 72
Route 3: 100
Route 4: 60
Route 24: 28
Route 25: 12
Route 5: 48
Route 6: 180
Route 11: 380
Route 9: 48
Route 10: 40
Route 8: 36
Route 7: 28
Route 16: 32
Route 17: 72
Route 18: 24
Route 12: 32
Route 13: 40
Route 14: 36
Route 15: 44
Route 19: 0
Route 21: 36
Route 20: 0
Route 23: 44
Route 27: 24
Route 26: 96
Route 28: 80
Johto
Route 29: 160
Route 46: 52
Route 30: 140
Route 31: 64
Route 36: 8
Route 32: 152
Route 33: 12
Route 34: 100
Route 35: 52
Route 37: 44
Route 38: 60
Route 39: 16
Route 40: 0
Route 41: 0
Route 42: 16
Route 43: 132
Route 44: 16
Route 45: 176
Route 47: 20
Route 48: 98
Hoenn
Route 101: 91
Route 103: 91
Route 102: 143
Route 104: 126
Route 116: 138
Route 110: 208
Route 117: 42
Route 111: 0
Route 112: 54
Route 113: 285
Route 114: 88
Route 115: 20
Route 118: 99
Route 119: 734
Route 120: 424
Route 121: 136
Route 123: 147
Routes 105-109, 122, and 124-134 are water routes, all with 0
Sinnoh
Route 201: 150
Route 202: 230
Route 203: 200
Route 204: 193
Route 205: 244
Route 206: 294 (unsure, perhaps more concealed by cycling road)
Route 207: 131
Route 208: 118
Route 209: 192
Route 210: 965
Route 211: 226
Route 212: 471
Route 213: 230
Route 214: 360
Route 215: 194
Route 216: 146
Route 217: 154
Route 218: 89
Route 219: 0
Route 220: 0
Route 221: 330
Route 222: 260
Route 223: 0
Route 224: 316
Route 225: 421
Route 226: 295
Route 227: 163
Route 228: 160
Route 229: 316
Route 230: 175
